1. Field of the Invention
The present invention relates to an optical pickup device and a method to control an angle made between a pit and a major axis of a laser beam, by which a superior reproduction signal feature can be obtained and by which a force corresponding to a birefringent disc is improved by controlling the pit and the major axis of the laser beam to form a predetermined angle.
2. Description of the Related Art
FIG. 1 shows an optical pickup device having an optical module 1 in which first and second laser light sources 3 and 5 emit first and second light beams I and II having different wavelengths and are integrally formed, as a technology relevant to the present invention. The optical pickup device includes the optical module 1, a beam splitter 10 reflecting or transmitting a light beam emitted from the optical module 1 to change an optical path, an objective lens 15 focusing the light beam reflected by the beam splitter 10 onto a relatively thin disc 17 or a relatively thick disc 18, and a photodetector 23 receiving and detecting the light beam that proceeds back after being reflected by the discs 17 and 18.
The optical module 1 has a mount 2 on which the first and second laser light sources 3 and 5 having different wavelengths are mounted. The first laser light source 3 is a laser diode emitting the light beam having a 650 nm wavelength, for example, and is used for the relatively thin disc 17, for example, a DVD. The second laser light source 5 is a laser diode emitting the light beam having a 780 nm wavelength, for example, and is used for the relatively thick disc 18, for example, a CD. These first and second laser light sources 3 and 5 are arranged to be separated about 110±2 μm from each other.
A grating 7 that divides the light beam emitted from the second laser light source 3 into three beams is provided on an optical path between the optical module 1 and the beam splitter 10, to detect a tracking error using a three-beam method to be described later.
The light beams reflected by the discs 17 and 18 are focused on the photodetector 23 after sequentially passing through the objective lens 15, a collimating lens 13, and the beam splitter 10. Here, a concave lens 20 may further be provided between the beam splitter 10 and the photodetector 23. The concave lens 20 is inclined in a direction opposite to an angle at which the beam splitter 10 is inclined to increase a size of a light spot formed on the photodetector 23, together with the collimating lens 13 and to remove coma aberration generated in the light beam passing through the beam splitter 10.
In the meantime, the light beams reflected by the discs 17 and 18 are formed on the photodetector 23. The photodetector 23, as shown in FIG. 2A, may be provided to correspond to each of the first and second laser light sources 3 and 5. For example, the photodetector 23 may be formed of a first photodetector 25 to detect the light beam emitted from the first laser light source 3 and a second photodetector 26 to detect the light beam emitted from the second laser light source 5. The first photodetector 25 is a four-section photodetector. The second photodetector 26 is formed of a four-section main optical sensor 26a and first and second sub-optical sensors 26b and 26c. 
As tracking error detection and focusing error detection are made by using signals detected by the first and second photodetectors 25 and 26, tracking and focus servo are performed.
A focusing servo of the first and second laser light sources 3 and 5 to the discs 17 and 18 is performed in an astigmatism method using a defocusing effect of an optical disc. A tracking servo of the first disc 17 is performed in a differential phase detection (DPD) method. The tracking servo of the second disc 18 is performed in a three-beam method. Here, the three-beam method is carried out by using the light beam which is diffracted by a grating 7 into three beams, reflected by the disc 18, and received and detected by the main and sub-optical sensors 26a, 26b, and 26c of the second photodetector 26.
A principle of reproducing information recorded on the disc 17 or 18 as the light beam passes along tracks formed on the disc by using the first and second photodetectors 25 and 26 is described below.
Referring to FIG. 2B, information is recorded in the form of a pit 33 that is formed along a track 30 of the respective discs 17 and 18. Reference numeral 35 denotes a first light spot which is formed by the light beam emitted from the first laser light source 3 focused on the respective discs 17 or 18. Reference numerals 36a, 36b, and 36c respectively denote a second main light spot and first and second side light spots which are formed by the light beam emitted from the second laser light source 5 split into three beams by the grating 7 and the three beams are focused on the respective discs 17 or 18.
However, because the first and second laser light sources 3 and 5 are arranged in the optical module 1, parallel to each other, the photodetectors 25 and 26 corresponding to the first and second laser light sources 3 and 5 are arranged parallel to each other. Because the first and second side light spots 36b and 36c are used for tracking, the first and second side light spots 36b and 36c are arranged parallel to a track direction T, which is a pit direction (Pit). Thus, as shown in FIG. 2A, a direction along which the first and second side light spots 36b and 36c are formed on the photodetectors 25 and 26 indicates the track direction T, and a direction perpendicular to the track direction T is a disc radial direction R.
As shown in FIG. 3A, the laser beams emitted from the first and second laser light sources 3 and 5 pass through a laser emission outlet 6 that is narrow. The laser beam is diffracted while passing through the laser emission outlet 6. As a size of the laser emission outlet 6 decreases, a diffraction angle increases. As shown in FIG. 3B, because the size of the laser emission outlet 6 in a horizontal direction M and a vertical direction L, through which the laser beam is emitted, are different, the laser beam is diffracted at different angles in the horizontal direction M and the vertical direction L. Here, the laser beam in the vertical direction L appears to be emitted from a point located at the front portion of the laser light source while the laser beam in the horizontal direction M appears to be emitted from a point located a distance ΔZ behind a front portion of the laser light source. The above path difference between the light beams in the horizontal direction and the vertical direction results in astigmatism. Here, θII denotes an angle spreading in the horizontal direction M and θ⊥ denotes an angle spreading in the vertical direction L.
Here, the vertical direction L is typically referred to as the major axis direction of the laser beam. The light beam emitted from the laser light sources 3 and 5, as shown in FIG. 4, has a major axis direction arranged parallel to a pit 33 on the disc 17 or 18. In FIG. 4, profiles of the first and second light beams I and II emitted from the laser light sources 3 and 5 are illustrated to be slightly exaggerated. However, it is widely known that, when the major axis of the laser beam does not make an angle of 45° with the pit, a defocus phenomenon occurs so that a signal characteristic deteriorates and a signal reproduction is difficult when the disc 17 or 18 has a large birefringence.
Thus, a variety of methods to form the light spot on the pit 33 such that the major axis direction of the light spot is angled by 45° with respect to the pit have been suggested. First, as one of simple methods, as shown in FIG. 5A, by rotating the optical module 3 and 5 by 45° such that the vertical direction L of the laser emission outlet 6 is rotated by 45°, the major axis of the light spot formed on the disc and the pit makes an angle of 45°. Here, when the laser light source 5 for the CD is rotated by 45° around the laser light source 3 for the DVD, the position of the light spot formed on the disc 17 or 18 is not affected. However, because the laser light source of the CD is rotated by 45° on the photodetector 23, the light reflected by the disc 17 or 18 is rotated by 45° and formed on the photodetector 23. Thus, as shown in FIG. 5B, by rotating the laser light source by 45°, the photodetector 23 is rotated by 45° to correspond to a changed position of the laser light source.
Here, the first and second sub-optical sensors 26b and 26c of the photodetector 23 should always be arranged parallel to the track direction T, that is, the direction of the pit 33, to enable tracking servo. When the photodetector 23 is rotated as above, because a side beam is not formed any more parallel to the track direction T on the photodetector 23, tracking cannot be performed. Thus, the above method cannot be a conclusive solution to have the major axis of the laser beam inclined by 45° with respect to the pit 33.